Methicillin-resistant Staphylococcus aureus (MRSA) infections are the most common cause of nosocomial or hospital-acquired infections (Archer, G. L. 1998. Clin. Infect. Dis. 26:1179). However, the incidence of MRSA infections has substantially increased over the last five years in healthy individuals without any known risk factors due to the worldwide emergence of distinct MRSA strains known collectively as community acquired methicillin resistant S. aureus (CA-MRSA) (e.g., Groom, A. V. et al. 2001. JAMA 286:1201-1205; Kazakova, S. V. et al. 2005. N. Engl. J. Med. 352:468-475; King, M. D. et al. 2006. Ann. Intern. Med. 144:309-317; Bratu, S. et al. 2006. Ann. Clin. Microbiol. Antimicrob. 5:29). CA-MRSA strains have become the most frequent cause of skin and soft tissue infections in emergency rooms in the U.S., increasing in proportion from 29% in 2001-2002 to 64% in 2003-2004 (King, M. D. et al. 2006. Ann. Intern. Med. 144:309-317; Moran, G. J. et al. 2005. Emerg. Infect. Dis. 11:928-930; Moran, G. J. et al. 2006. NEJM 355:666-674). CA-MRSA strains appear to have evolved independently of hospital-acquired MRSA (HA-MRSA) and are genetically distinct from HA-MRSA. Moreover, many strains of CA-MRSA are more virulent than HA-MRSA, causing a different spectrum of symptoms and effects, which include necrotizing fasciitis and pneumonia in otherwise healthy individuals, but also carrying with it a different antibiotic resistance profile. CA-MRSA is now the most common cause of skin infections in the United States. The epidemiological characteristics of CA-MRSA and HA-MRSA are therefore distinct from each other and CA-MRSA requires attention as an independent and increasingly important public health problem.
USA300 and MW2 (USA400), representing two distinct isolates by pulsed-field electrophoresis, are the predominant strains of CA-MRSA in the U.S. (Groom, A. V. et al. 2001. JAMA 286:1201-1205; Baba, T. et al. 2002. Lancet 359:1819-1827). Although initially seen in cutaneous infections, USA300, for example, has now become a major cause of sepsis and prosthetic joint infections with limited therapeutic options (Seybold, U. et al. 2006. Clin. Infect. Dis. 42:647-656; Kourbatova, E. V. et al. 2005. Am. J. Infect. Control 33:385-391; Gales, A. C. et al. 2006. Int. J. Antimicrob. Agents 27:300; Skiest, D. J. 2006. J. Clin. Microbiol. 44:655-656). Although HA-MRSA and CA-MRSA S. aureus strains have a similar core genome, they do carry substantial genetic and phenotypic differences which may explain why CA-MRSA strains are significantly more virulent in a mouse model of S. aureus infection, with a greater level of pathology in major vital organs, more resistant to killing by human PMNs and capable of causing greater host cell lysis (Voyich, J. M. et al. 2005. J. Immunol. 175:3907-3919). Thus, there is a need to understand and define the basis for CA-MRSA apart from HA-MRSA, including the potential treatments that may be unique for each type of MRSA. There is currently no reliable method to quickly and accurately diagnose CA-MRSA, distinguishing it from HA-MRSA, as well as a diminishing number of options for treatment of CA-MRSA infections.
Resistance to a greater number of antibiotics has occurred in S. aureus isolates worldwide. Besides common resistance to methicillin and β-lactams in general, S. aureus has also become resistant to drugs of last resort such as vancomycin, linezolid and daptomycin (Gales, A. C. et al. 2006. Int. J. Antimicrob. Agents 27:300-302; Skiest, D. J. 2006. J. Clin. Microbiol. 44:655-656; Howden, B. P. et al. 2004. Clin. Infect. Dis. 39:1544; Ruiz, M. E. et al. 2002. Clin. Infect. Dis. 35:1018-1020; Saner, F. H. et al. 2006. Liver Transpl. 12:1689-1692; Hirschwerk, D. et al. 2006. Infect. Control Hosp. Epidemiol. 27:315-317). All S. aureus isolates, both methicillin sensitive and resistant strains, carry three high molecular weight penicillin binding proteins, PBP1, PBP2 and PBP3 to which most β-lactam antibiotics bind, and a low molecular weight PBP called PBP4 which binds poorly to most β-lactams. PBP1 and PBP2 are essential enzymes involved in the synthesis of bacterial cell wall; the β-lactam antibiotics generally kill bacteria by interfering with the transpeptidase domain of penicillin binding proteins (PBPs), which leads to a loss of cell-wall cross-linking and integrity (Mallorqui-Fernandez, G. et al. 2004. FEMS Microbiol. Lett. 235:1-8). PBP4, the single low molecular weight PBP, has been shown to have a low affinity for most β-lactams, and is unique among low-molecular weight PBPs found among prokaryotes in that it possesses transpeptidase and carboxypeptidase activities (Kozarich, J. W. and J. L. Strominger. 1978. J. Biol. Chem. 253:1272-1278; Georgopapadakou, N. H. and F. Y. Liu. 1980. Antimicrob. Agents Chemother. 18:834-836; Georgopapadakou, N. H. and F. Y. Liu. 1980. Antimicrob. Agents Chemother. 18:148-157; Georgopapadakou, N. H. et al. 1982. Antimicrob. Agents Chemother. 22:172-175).
Methicillin resistance is achieved by acquisition of another high molecular weight PBP, namely PBP2A encoded by mecA; this is situated in the chromosome in a genomic island designated staphylococcal cassette chromosome mec (SCCmec). Unlike innate penicillin binding proteins, PBP2A has a remarkably low affinity for all β-lactams (Matsuhashi, M. et al. 1986. J. Bacteriol. 167:975).
In one study, PBP4 was shown to play a role in the synergistic effect of combining imipenem, an autolytic agent, with cefotiam, a compound with no affinity for PBP4, against MRSA; the synergistic activity of the two agents against MRSA was correlated with the degree of autolysis induced by imipenem (Matsuda, K. et al. 1995. Antimicrob. Agents Chemother. 39:2631-2634). PBP4 expression levels have been shown to affect vancomycin susceptibility in S. aureus strains COL, RN450M and N315 (Finan, J. E. et al. 2001. Antimicrob. Agents Chemother. 45:3070-3075). Previous studies also have linked loss of PBP4 to a drastic reduction in peptidoglycan cross-linking in MSSA, MRSA COL and also glycopeptide-resistant S. aureus (Sieradzki, K. et al. 1999. J. Biol. Chem. 274:18942-18946). However, in a recent study, a deletion of the PBP4 gene in the HA-MRSA prototypic strain COL was not lethal and had relatively little effect on β-lactam resistance (Katayama, Y. et al. 2003. Microb. Drug Resist. 9:329-336); the authors, therefore, concluded that PBP4 is a relatively unimportant target for β-lactam antibiotics in MRSA as well as methicillin-susceptible S. aureus. 
As a consequence of the differences in epidemiology, virulence, and antibiotic susceptibility, there remains a need to be able to distinguish quickly and accurately between CA-MRSA and HA-MRSA infections. Without such distinction, patients are at risk of being improperly or tardily diagnosed, and/or receiving ineffective treatments and developing more severe infection, situations which can result in dire consequences for the patient, including death